Aspergillus is one of the most common fungal pathogens affecting neutropenic patients and other types of immunocompromised individuals such as those with Chronic Granulomatous Disease(CGD) of Childhood. Among a dozen species of Aspergillus reported to cause infection in humans, A. fumigatus is the most common species reported to cause invasive aspergillosis. All Aspergillus species propagate by conidia (spores), which humans encounter daily through inhalation. During 2007-2008, we investigated the Aspergillus response to host immune cells by studying the genes differentially expressed in conidia vs hyphae when challenged with neutrophils from healthy donors as well as from those with chronic granulomatous disease (CGD). To our knowledge, this is the first study that investigated the genes differentially expressed in conidia vs hyphae of A. fumigatus in response to neutrophils from healthy donors as well as from those with CGD which are defective in the production of reactive oxygen species. Upon exposure to either normal or CGD neutrophils, 244 genes were up-regulated in conidia but not in hyphae. Several of these genes are involved in the degradation of fatty acids, peroxisome function and the glyoxylate cycle which suggests that conidia exposed to neutrophils reprogram their metabolism to adjust to the host environment. In addition, the mRNA levels of four genes encoding proteins putatively involved in iron/copper assimilation were found to be higher in conidia and hyphae exposed to normal neutrophils compared to those exposed to CGD neutrophils. Deletants of several of these differentially expressed genes showed phenotypes related to the proposed functions, i.e. deletants of genes involved in fatty acid catabolism showed defective growth on fatty acids and the deletants of iron/copper assimilation showed a higher sensitivity to the oxidative agent menadione. None of these deletants, however, showed reduced resistance to neutrophil attack. During 2008-2009, we characterized the differences in disease manifestations and the manner of dissemination beween A. fumigatus and its sister species, A. udagawae. Both A. udagawae (Neosartorya udagawae) and A. fumigatus belong to Section Fumigatii in the genus Aspergillus and they can not be differentiated by their morphological characteristics but can be differentiated by genetic methods. We also characterized the biological differences between the two species.During 2009-2010, we focused on the identification of MAT-1 and MAT-2 strains that could be used as the foundational strains for the construction of an isogenic pair for the genetic study of virulence factors and other traits of pathobiological importance. We found two super maters of the opposite mating type and decided to use the AFB62 strain that originated from a case of invasive aspergillosis. During 2010-2011, we characterized the sexual reproduction by the super mater pair which was chosen for the construction of the MAT-1 and MAT-2 isogenic set. The meiotic product (ascospores) of the pair could be readily purified from contaminating mitotic spores by heat treatment for 30 minutes at 70 C. High recombination frequencies obtained by mating two conidial color mutants derived from the super mater pair:MAT-1 carrying abr2 mutation, and MAT-2 carrying alb1 mutation and the mutated genes were located 19 kb apart on the same chromosome, indicated that the pair can be used as a genetic tool for recombinational analysis. During 2011-2012, we discovered a new pathogenic species of Aspergillus phylogenetically unrelated to A. fumigatus that causes invasive aspergillosis refractory to currently available antimycotic agents. The new species was described as A. tanneri belonging to the Aspergillus section Circumdati.The species was innately resistant to polyenes, azoles and echinocandins and caused fatal infections in CGD patients.During 2012-2013, we compared the virulence of A.tanneri with that of A. fumigatus in various host systems including CGD mice, corticosteroid treated mice and Galleria mellonela and found that the new species is significantly more virulent for Galleria but less virulent for the mice models. During 2013-2014, we discovered three new species A. fumigatus-look alike species that are genetically and pathobiologically different from A. fumigatus.